VSE problem blurring license plate - mask is jumping all over video
It's likely that you're running into the problem between Interframe Compression vs. Intraframe Compression.

Blender (and other video editors) often have trouble scrubbing through video that has been compressed referentially. In this type of compression, one frame is encoded as the changes from the previous frame, so all the data isn't there for every frame. This means, it's really easy to get lost between keyframes. I've seen this problem when I'm tracking video it seems like the track is really bad, and jumping all over the place, but what's really going on is the frame you're seeing isn't the right frame for that time signature.

Try converting to an Intraframe format (each full frame is compressed individually) like a JPG image sequence in an AVI container. Blender can do that conversion pretty easily.